DESCRIPTION: Vertebrate learning is one of the major problems of neurobiology. Progress depends on experimental models in which the plasticity underlying simple learning occurs in defined and accessible regions. Studies from this laboratory have demonstrated operant conditioning of the simplest behavior of the vertebrate CNS, the H- reflex, which is the electrical analog of the spinal stretch reflex. Once an animal has been conditioned, the reflex change persists even if all supraspinal input is removed. Thus, the responsible plasticity is in the spinal cord, and H-reflex conditioning is a powerful model for studying the neuronal and synaptic substrates that underlie a learned change in behavior. In addition, it is the basis for a promising new therapeutic approach to spasticity and other forms of abnormal reflex function. The goal of this project is to define the changes in the spinal cord that are responsible for operantly conditioned change in the H-reflex. The central hypotheses, based on the results of the past grant period, are : 1) that H- reflex decrease is due to a positive shift in motoneuron firing threshold combined with a decrease in the Ia afferent EPSPs of fast motoneurons, and 2) that H-reflex increase is due to a change in short-latency polysynaptic input to the motoneuron. To test these hypotheses, rats in which the triceps surae H-reflex has been increased or decreased by operant conditioning will be studied. Intracellular recordings from motoneurons will define their intrinsic properties, motor unit type, and monosynaptic and polysynaptic responses to afferent inputs. Comparison of data from conditioned and naive rats will reveal the plasticity produced by H-reflex conditioning, and further analysis will examine the relationships between this plasticity and behavioral change. The ultimate objective is to define fully the adaptive plasticity produced by H-reflex conditioning and the translation of that plasticity into behavior. The project should lead to new understanding of the plasticity that underlies learning in vertebrates.